Move each dart GN rule to a standalone file.

[matanlurey]

We plan to add more rules in the near future, i.e. dart_executable, dart_test.

It would be nice to have a several thousand line scary file, and a few smaller, somewhat less scary, files.

At a meta level, part of flutter/flutter#147013.

[cbracken]

Looks like a few builds are blowing up:

ERROR at //flutter/build/dart/internal/flutter_snapshot.gni:24:7: Undefined identifier
      flutter_runtime_mode == "profile" || flutter_runtime_mode == "release"
      ^-------------------
See //flutter/testing/scenario_app/BUILD.gn:8:1: whence it was called.
flutter_snapshot("scenario_app_snapshot") {
^------------------------------------------
See //flutter/BUILD.gn:156:22: which caused the file to be included.
    public_deps += [ "//flutter/testing/scenario_app" ]

[cbracken]